1. Technical Field
The present invention relates to a flat surface illumination device and a liquid crystal display panel capable of compensating and preventing warp caused by change of temperature, humidity and the like of a light guide.
2. Prior Art
In the conventional flat surface illumination device, a line light source, a light guide, a reflecting sheet and the like have been accommodated in a plastic case and, in some cases, an aperture has been formed by cutting off a part of the plastic case for lightening weight thereof. And, a frame or a case with a reinforcing frame disposed in a cross shape on the back of the light guide has been used.
A variety of kinds of light guides have been used. However, a flat plate with a little warp and a uniform thickness or a wedge-shaped light guide manufactured by casting process has been used when warp of a light guide is required to be decreased as much as possible, e.g., in a large flat surface illumination device of 13 inch or more.
In Japanese Patent Laid-Open Hei 7 (1995)-318936, a technology for diffusing heat uniformly, decreasing luminance unevenness caused by a reflecting sheet and protecting the reflecting sheet by disposing a metal plate on the back of the light guide is disclosed. It is also disclosed that a metal plate may be disposed near or on only a part outside a reflecting sheet.
It has been found that in case that a part of a plastic case is cut off, the light guide is supported only by a circumferential section, so that warp of the light guide is caused by weight itself.
In case of a large flat surface illumination device of 13 inch or more, increase of the length in a longitudinal direction causes warp in the longitudinal direction. In this case, a flat plate with a uniform thickness has been used in order to decrease warp of a light guide as much as possible. However, as a display area size of a liquid crystal display device used in a portable personal computer and the like has been recently enlarged, needs for lightening weight of a flat surface illumination device has occurred and a problem of weight has been presented. A method of manufacturing a flat plate with a uniform thickness is generally a method of cutting from a large resin plate. This method presents a problem in productivity. Moreover, when a conventional flat surface illumination device using a large wedge-shaped light guide for lightening weight thereof is manufactured by a casting process, the problem is presented that it takes long time for the whole to be cured after basic material resin and curing agent are injected into a metallic mold of a wedge-shaped light guide. In some cases, a flat plate or a casting light guide of this kind can not completely prevent warp.
Furthermore, in a method of preventing warp from the back side of a light guide with a reinforcing frame disposed in a cross shape in a frame or a case, in some cases, the whole warps in the same direction because of the same material and at the same time mechanical strength depends on the thickness of a resin. In this method, the problem for light weight and strengthening is presented. Moreover, especially to enlarge a flat surface illumination device, a body of a frame or a case is tapered according to a shape of a wedge-shaped light guide. In this case, the problem for strength is presented.
A flat surface illumination device of the present invention comprises a line light source, a light guide guiding a light ray from the line light source, a plastic frame with an aperture supporting the line light source and the light guide, a warp preventing means made of inorganic material attached to the plastic frame in such a manner that it crosses the aperture. The aperture is formed so as to include a part corresponding to a position of center of gravity of the light guide. The area of the aperture is 25xc3xa7 or more, 50xc3xa7 or more, or 90xc3xa7 or more of the area of the light guide surface adjacent to the plastic frame in the light guide. A warp preventing means is, e.g., metallic material or ceramic material. A flat surface illumination device of the present invention may include at least one optical sheet in a group constituting a light condensing sheet, a light reflecting sheet, a light diffusing sheet and a light polarizing sheet.
A flat surface illumination device of the present invention can prevent warp of a light guide by inserting a warp preventing means made of metallic material, in the longitudinal direction from an incident end surface section to a reflecting end surface section, between the back side of the light guide and the frame or the case.
In a flat surface illumination device of the present invention, the warp preventing means comprises a long and narrow zonal thin plate, corresponding to the length in a longitudinal direction of the light guide, in which at least two beading processes are performed in the longitudinal direction and/or a perpendicular bending process is performed at both ends in the longitudinal direction. The warp preventing means makes it possible to produce a light and thin flat surface illumination device with an adequate mechanical strength.
In a flat surface illumination device of the present invention, a step-difference process is performed in at least one of the ends of the zonal thin plate in the longitudinal direction. Accordingly, it is possible to produce a light and thin flat surface illumination device with an adequate mechanical strength as well as easily attachable to a frame or a case.
In a flat surface illumination device of the present invention, a beading process is performed in the ratio of the width processed by the beading to the width of the zonal thin plate ranging from 1:2.3 to 1:5. Accordingly, it is possible to freely select a required mechanical strength according to the allowable width of the zonal thin plate.
In a flat surface illumination device of the present invention, a bending process where the length of a perpendicularly bent section is 1 to 5 times as large as the thickness of the zonal thin plate is used. Accordingly, it is possible to freely select a required mechanical strength according to the allowable width of the zonal thin plate.
In a flat surface illumination device of the present invention, a plastic frame or case comprises a concavity accommodating an end of a zonal thin plate of the warp preventing means in a position corresponding to an incident end surface section and a reflection surface end section of the light guide and/or an aperture into which the end of the zonal thin plate is inserted. Accordingly, it is possible for the zonal thin plate of the warp preventing means to be fixed and removed easily and for the number of zonal thin plates of the warp preventing means to be adjusted according to use.
A flat surface illumination panel of the invention comprises a line light source, a light guide guiding a light ray from the line light source, a liquid crystal display element selectively transmitting the light ray from the light guide, a plastic frame with an aperture supporting the line light source and the light guide, a warp preventing means made of inorganic material attached to the plastic frame in such a manner that it crosses the aperture. A liquid crystal display panel of the invention comprises a line light source, a light guide guiding a light ray from the line light source, a liquid crystal display element selectively transmitting light ray from the light guide, a plastic frame, with an aperture, supporting the line light source and the light guide, a metal bezel supporting the plastic frame, a warp preventing means made of inorganic material attached to the metal bezel in such a manner that it crosses the aperture. A liquid crystal display device of the invention comprises a line light source, a light guide guiding a light ray from the line light source, a liquid crystal display element selectively transmitting the light ray from the light guide, a plastic frame, with an aperture, supporting the line light source and the light guide, a metal bezel supporting the plastic frame, an outer lid supporting the metal bezel, a warp preventing means made of inorganic material attached to the outer lid in such a manner that it crosses the aperture.